Biology Reference
In-Depth Information
therefore also be possible to incorporate other nonconventional monomers
into pine lignin to facilitate processing.
1. Incorporation of syringyl units into conifer lignins
Most hardwood species contain a lignin polymer that is rich in syringyl (S)
units, producing a lignin type not found in conifer species such as pine (
Baucher
et al.,1998
). The advantage of S-type monomers over H- and G-type mono-
mers is that they do not generate the same degree of condensation in the lignin
polymer. In addition, a lignin polymer rich in S-units contains more
-ethers
that more easily degrade using alkaline pulping or acidolytic methods (
Brunow
and Lundquist, 2010; Ralph, 2010
). The resulting lignin polymer is consequent-
lymore easily removed frompolysaccharide components of the cell wall matrix.
This explains why increasing the proportion of syringyl lignin in angiosperm
species improved pulping and biofuel production of lignocellulosic material
(
Huntley et al., 2003; Stewart et al.,2009;Studeret al.,2011
). S-lignin in
angiosperms is naturally very abundant in wood fibres and is less prominent
in vessel elements, which are rich in G-lignin (
Harris, 2006
). However, recom-
binant lignin studies in angiosperm species such as A. thaliana, Nicotiana
tabacum and Populus tremula x alba have proven that it is possible to increase
S content in the lignin polymer to over 95%without compromising plant fitness
or performance (
Franke et al., 2000; Huntley et al.,2003;Liet al.,2003;Sibout
et al.,2002;Stewartet al.,2009
). The S-rich lignin produced in transgenic
angiosperms seems to enable all wood-related cell types to function normally,
including the naturally G-rich vessel elements. It is consequently conceivable
that a lignin polymer containing S-units also enables pine tracheids to function
normally. The introduction of S-lignin into conifers could substantially im-
prove utilisation and processing of biomaterials derived from conifers.
Production of S-lignin in conifers would, at a minimum, require the
introduction of two key enzymes involved in the biosynthesis of sinapyl
alcohol, CAld5H (originally termed F5H), and CAldOMT (originally termed
COMT) (
Fig. 8
). Both enzymes are essential for the production of S-lignin in
angiosperms (
Humphreys et al., 1999; Osakabe et al., 1999
) and are absent in
conifer species. The production of S-lignin has evolved in different plant
lineages via convergent evolution. Angiosperms and lycophytes have inde-
pendently developed the capability to synthesize sinapyl alcohol (
Weng et al.,
2010, 2011
). CAld5H enzymes from lycophytes and angiosperms differ in
their catalytic properties (
Weng et al., 2010
). This opens up the opportunity
to test CAld5H genes with different enzymatic properties in a conifer back-
ground. CAld5H from lycophytes has the advantage over the angiosperm
CAld5H of being able to accept 3-hydroxylate p-coumaraldehyde as well as
5-hydroxylate coniferaldehyde as substrate. This could potentially promote
b
